The present invention relates generally to the bonding of an interposer with a lead frame in semiconductor packaging applications. More particularly, an improved method and apparatus for maintaining contact between the interposer and lead frame during single point bonding is disclosed.
In semiconductor packaging, lead frames are commonly used to couple a die to external components. In some applications (particularly small high lead count packages), it may be desirable or necessary to use an interposer in conjunction with the lead frame. The interposer is similar to a mini printed circuit board having traces. The lead frame is ultrasonically bonded directly onto underlying traces on an interposer, and the bonding pads on the dies are electrically coupled to the other end of the trace using bonding wires. One end of each bonding wire is typically ultrasonically bonded to a die pads on the die and the opposite end is typically bonded to an associated lead.
During an ultrasonic bonding process, the quality of the bond formed between the bonding surfaces is a major concern. This concern is especially acute during lead frame-interposer integration where high power bonding is necessary. High power is required to transfer the ultrasonic energy through the thick lead to the underlying interposer trace. In high power bonding, it is difficult to maintain contact between the bonding surfaces, the lead and the interposer trace. Any deviation in the quality of the contact between the bonding surfaces directly affects the bond quality.
Referring to FIG. 1, a conventional approach to providing a lead frame clamp during lead frame-interposer integration, generally designated by reference numeral 100, will be described. The lead frame clamp 100, which is in the form of a window clamp, includes a contact section 16 having a window hole 18, two support sections 14, and a plurality of fasteners. Each fastener is in the form of a fastener hole 12 and a screw or rivet 13. During the bonding process, screws or rivets 13 are inserted into the fastener holes 12, and the fasteners are attached to and support the support sections 14 in a fixed position. The support sections 14 are attached to and support the contact section 16 in a fixed position so that the contact section 16 touches and applies pressure to an area of the lead frame that includes the lead tips. A portion of the lead tip ends are left exposed by the window hole 18 and are not covered by the contact section 16. While pressure is being applied by the contact section 16 to the lead frame and underlying interposer, the exposed lead tips are bonded to the traces on the interposer.
Unfortunately, these conventional lead frame clamps possess certain drawbacks. First, the amount of movement of a lead is higher for a lead with an axis that lies in a direction perpendicular to the bonding tool's movement than for a parallel lead. This difference in movement results in higher attenuation of the ultrasonic energy for the perpendicular lead than for the parallel lead. This difference in attenuation dictates that the optimal bonding power for the parallel leads is greater than the optimal bonding power for the perpendicular leads. However, when the bonding power is high, and ultrasonic energy is applied for a longer period than is necessary to facilitate bonding, the interposer traces are more likely to break or the bond itself may be destroyed during bonding. Second, since the perpendicular leads easily move during bonding, the resulting bond quality is adversely affected. Third, if the bonding power is high enough, the vibration resonance on previously bonded points may be high enough to destroy these bonds. These three problems illustrate the need to improve the stability of the lead frame and the interposer during the bonding process. Thus, an improved clamping mechanism would be desirable.